Here's a great tool for checking all of your e-mail accounts, whether they're Web-based like Gmail, or POP 3 accounts, such as is common among e-mail providers. It works with just about any Web-based mail or ISP, including Gmail, Hotmail, Yahoo Mail, Juno, and many others.
After a brief setup for each account, POP Peeper checks your mail regularly, and then alerts you when new mail comes in. You can get video and audio alerts, and you can also customize the sounds so that you get a different sound for each of your accounts. It's small enough so that you can use it as portable software--install it on a USB thumb drive and carry it with you.
POP Peeper does more than just alert you when there's new mail. It's also an e-mail reader and lets you create new mail and delete mail as well. You can even send or open file attachments with it.